The use of pigmented layers as a coloring coating for transparent glass or glass ceramic substrates, which are applied as a bottom surface coating for cooktops, for example, is known from prior art. Frequently, pigmented sol-gel coatings are used for this purpose.
Depending on the use of the coated substrates, the coloring layers have to meet specific requirements. If used as a bottom surface coating of cooktops, for example, they not only have to comply with adhesive strength, scratch resistance, thermal stability, and other functional criteria but also have to exhibit a minimum imperviousness to a variety of media, which may come into contact with the coated cooktop from below. Though coloring sol-gel based decoration layers indeed exhibit a certain imperviousness, this is however not nearly sufficient to meet the requirements of cooker manufacturers.
One way to obtain a coloring coating that meets the above criteria and also achieves sufficient imperviousness is described in detail in patent application DE102009004783A1. Here, the decoration layer is covered by a sealing layer which comprises another sol-gel layer. Both sol-gel layers together form a decorative coating. The composition of the sealing layer may be similar to the composition range of the coloring layer, or may differ therefrom.
In many cases, this layer system comprising two sol-gel layers of defined compositions meets the requirements. However, in order to satisfy the requirements on thermal stability, adhesive strength and imperviousness, only specific pigments must be used in defined proportions.
Momentary overheating of the cooktop may cause discoloration of the decorative coating. Solvents from adhesives or silicone oil containing contact media may lead to insufficient imperviousness, so that e.g. the adhesive beads which serve to fix the cooking hobs to the cooktops become visible. An adequate sealing effect can only be achieved with very specific matrix-pigment ratios and by adding a solid lubricant.
Moreover, various colorimetric loci, such as those of “black”, but also very light-colored colorimetric loci, cannot be reproduced by the pigmentations of the decorative coating described. Other colorimetric loci may be provided but with poor color stability and poor sealing functionality.